Copolymers of vinylidene fluoride (VDF) with vinyl alkoxy sulfonyl halides are known in the art.
The disclosures in Ezzell et al. (U.S. Pat. No. 4,940,525) encompass copolymers of VDF with vinyl ethoxy sulfonyl fluorides containing one ether linkage. Disclosed is a process for emulsion polymerization of tetrafluoroethylene (TFE) with the vinyl ethoxy comonomer.
Banerjee et al. (U.S. Pat. No. 5,672,438) disclose copolymers of VDF with vinyl alkoxy sulfonyl fluorides containing more than one ether linkage.
Connolly et al. (U.S. Pat. No. 3,282,875) disclose the terpolymer of VDF with perfluorosulfonyl fluoride ethoxy propyl vinyl ether (PSEPVE) and hexafluoropropylene (HFP). They broadly teach an emulsion polymerization process said to be applicable to copolymerization of vinyl ethers with any ethylenically unsaturated comonomer, with greatest applicability to fluorinated monomers.
Barnes et al. (U.S. Pat. No. 5,595,676) disclose "substantially fluorinated" copolymers of a vinyl ether cation exchange group-containing monomer with a "substantially fluorinated" alkene. The copolymer is produced by controlled addition of the alkene in emulsion polymerization, followed by hydrolysis in NaOH. PSEPVE/TFE copolymers are exemplified.
Hietala et al., J. Mater. Chem. Volume 7 pages 721-726, 1997, disclose a porous poly(vinylidene fluoride) on to which styrene is grafted by exposing the PVDF to irradiation. The styrene functionality is subsequently functionalized to sulfonic acid by exposure of the polymer to chlorosulfonic acid. The resultant acid polymer, in combination with water, provides a proton-conducting membrane.
Formation of ionomers and acid copolymers by hydrolysis of the sulfonyl fluoride functionality in copolymers of TFE and fluoro alkoxy sulfonyl fluorides is well known in the art. The art teaches exposure of the copolymer to strongly basic conditions.
See for example, Ezzell et al. U.S. Pat. No. 4,940,525, wherein is used 25 wt % NaOH(aq) for 16 hours at 80-90.degree. C.; Banerjee et al. U.S. Pat. No. 5,672,438, wherein is used 25 wt % NaOH for 16 hours at 90.degree. C., or, in the alternative, an aqueous solution of 6-20% alkali metal hydroxide and 5-40% polar organic liquid (e.g., DMSO) for 5 minutes at 50-100.degree. C.; Ezzell et al. U.S. Pat. No. 4,358,545 wherein is used 0.05N NaOH for 30 minutes for 50.degree. C.; Ezzell et al. U.S. Pat. No. 4,330,654, wherein is used 95% boiling ethanol for 30 minutes followed by addition of equal volume of 30% NaOH (aq) with heating continued for 1 hour; Marshall et al. EP 0345964 A1, wherein is used 32 wt % NaOH (aq) and methanol for 16 hours at 70.degree. C., or, in the alternative, an aqueous solution of 11 wt % KOH and 30 wt % DMSO for 1 hour at 90.degree. C.; and, Barnes et al. U.S. Pat. No. 5,595,676, wherein is used 20 wt % NaOH (aq) for 17 hours at 90.degree. C.
Because of its high dielectric constant, high electrochemical stability, and desirable swelling properties, poly(vinylidene fluoride) is known in the art of lithium batteries as a highly desirable material for use as a membrane separator. For example Gozdz et al. (U.S. Pat. No. 5,418,091) disclose porous PVDF homopolymer and copolymer containing solutions of lithium salts in aprotic solvents useful as separators in lithium batteries.
Porous membranes of the type described by Gozdz, however, conduct both the cation and the anion back and forth across the separator, and are thus subject to concentration polarization during use, which degrades the performance of the battery in which it is used. So-called single ion conducting polymeric membranes, wherein the ionic salt is attached to the polymer backbone, thereby immobilizing either the cation or the anion, offer a solution to the concentration polarization problem, and are known in the art. One particularly well-known such single ion conducting polymer is Nafion.RTM. Perfluoroionomer Resin and Membranes available from DuPont, Wilmington, Del. Nafion is a copolymer of TFE and perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride) which has been hydrolyzed by treatment with an alkali metal hydroxide according to the teachings of the art as hereinabove described.
It is further known in the art, and hereinbelow shown, that PVDF homopolymers and copolymers are subject to attack by strong bases such as the alkali metal hydroxides taught in the art hereinabove cited. Of particular importance is that the attack of basic nucleophiles on a copolymer of VDF and perfluorovinyl ethers results in the removal of the vinyl ether moiety from the polymer, see W. W. Schmiegel in Die Angewandte Makromolekulare Chemie, 76/77 pp 39ff, 1979. Since the highly preferred monomer species taught in the art, and exemplified by DuPont's Nafion and similar products, for imparting ionomeric character to various polymers is a vinyl ether terminated by a sulfonyl halide functionality, the sensitivity to base attack of the VDF copolymer formed therewith has prevented the development of a single-ion conducting ionomer based upon VDF. There simply is no means taught in the art for making the ionomer.